McCain, a staunch foreign policy hawk, said Thursday that Paul's warnings that the U.S. could target "Jane Fonda" or "people in cafes" bring the debate into the "realm of the ridiculous."

"If Mr. Paul wants to be taken seriously he needs to do more than pull political stunts that fire up impressionable libertarian kids," McCain said, adding: "I don't think what happened yesterday is helpful to the American people."